The present invention relates to a sheet turn-over apparatus attached to a sheet-fed rotary printing press to turn over a sheet having a printed upper surface so as to print an image on the lower surface of the sheet.
A sheet-fed rotary printing press for performing single side printing and double side printing is commercially available due to a variety of printing modes. A sheet turn-over apparatus is attached to the sheet-fed rotary printing press to grip the trailing edge of the sheet having a printed upper surface and turn over the sheet in double side printing.
FIG. 13 shows a cylinder layout for explaining double side printing in a three-cylinder printing press including a transfer cylinder, a turn-over cylinder, and a double-diameter cylinder, all of which constitute a turn-over mechanism. The construction and double side printing of the printing press will be described with reference to FIG. 13. An impression cylinder 5 having gripper units 4 each comprising a gripper 2 and a gripper pad 3 is located obliquely below a blanket cylinder 1 which is located below a plate cylinder (not shown) and is in rolling contact therewith. A blanket cylinder 6 held in rolling contact with a plate cylinder and an impression cylinder 10 having gripper units 9 each comprising a gripper 7 and a gripper pad 8 and held in rolling contact with the blanket cylinder 6 are arranged in the downstream printing unit. A turn-over cylinder assembly consisting of a transfer cylinder 11, a double-diameter cylinder 12, and a turn-over cylinder, all of which are held in rolling contact with each other, is arranged between the impression cylinders 5 and 10. Of these cylinders for turn-over operation, the transfer cylinder 11 comprises gripper units 16 each comprising a gripper 14 and a gripper pad 15 in the axial gap on the outer surface. The double-diameter cylinder 12 has a diameter twice that of the transfer cylinder 11. Gripper units 19 and 20 each comprising a gripper 17 and a gripper pad 18 are arranged in the gaps of the outer surface at equal angular intervals. Two sets of suction heads 21 and 22 are axially arranged at forward positions (with respect to the rotational direction) spaced by predetermined angular intervals from the gripper units 19 and 20, respectively. More specifically, a plurality of suction heads 21 (or 22) are axially arranged in the outer surface of the cylinder 12 and are circumferentially movable, thereby adjusting the circumferential phase of the gripper units 19 (or 20). Gripper units (A) 25 each comprising a gripper 23 and a gripper pad 24 and gripper units (B) 28 each comprising a gripper 26 and a gripper pad 27 are circumferentially adjacent with each other and arranged in axial gaps, respectively. The cylinders 1, 5, 11, 12, 13, 10, and 6 are coupled by gears and are rotated in directions indicated by arrows.
With the above arrangement, in double side printing, the double-diameter cylinder 12 is rotated while the suction heads 21 and 22 are fixed and the phases of the gripper units 19 and 20 for the suction heads 21 and 22 are adjusted according to the paper size. Thereafter, all the cylinders are rotated in the directions indicated by the arrows, and a sheet 30 is gripped by the gripper units 4 of the impression cylinder 5. When the sheet 30 passes between the blanket cylinder 1 and the impression cylinder 5, an image is printed on the upper surface of the sheet 30 by the blanket cylinder 1. The sheet 30 is then regripped from the gripper units 4 of the impression cylinder 5 to the gripper units 16 of the transfer cylinder 11 and is conveyed by the transfer cylinder 11. The sheet 30 is regripped from the gripper units 16 of the transfer cylinder 11 to the gripper units 19 (or 20) of the double-diameter cylinder 12 and is conveyed while being chucked by the suction heads 21 (or 22). When the sheet 30 is wound around the double-diameter cylinder 12, and the trailing edge of the sheet reaches a point of contact between the cylinders 12 and 13, the suction heads 21 (or 22) oppose the gripper units (B) 28, and suction air is removed from the suction heads 21 (or 22). In this state, the gripper units (B) 28 are closed and the trailing edge of the sheet is released from the suction heads 21 (or 22) and is gripped by the gripper units (B) 28. When the sheet is directed toward the impression cylinder 10 while being gripped by the gripper units (B) 28, the direction of the gripper units (B) 28 is opposite to that of the gripper units 9, and transfer of the sheet from the gripper units (B) 28 to the gripper units 9 cannot be performed. The sheet 30 is regripped by the gripper units (A) 25 having a direction opposite to that of the gripper units (B) 28 and is conveyed by the gripper units (A) 25. In this manner, the sheet 30 which has turned over is regripped by the gripper units 9 of the impression cylinder 10. The sheet 30 passes between the blanket cylinder 6 and the impression cylinder 10. In this case, the lower surface of the sheet is brought into contact with the blanket cylinder 6, so that an image is transferred from the blanket cylinder 6 to the lower surface of the sheet, thereby completing double side printing.
When printing is started after the phase of the upstream cylinders including the double-diameter cylinder 12 is shifted with respect to the turn-over cylinder 13 and the gripper units 19 are set at positions corresponding to that of the gripper units (A) 25, the sheet 30 is not turned over and single side printing is performed. In this case, the gripper units (B) 28 are kept inoperative.
In the printing press operated as described above, during sheet turn-over operation in double side printing, the trailing edge of the sheet must be regripped from the gripper units (B) 28 to the gripper units (A) 25. In this case, the gripper units (A) 25 and (B) 28 whose gripper tips are directed outward must be turned over so as to regripper the sheet to the gripper units 19 and 9. A conventional sheet turn-over apparatus of this type includes a gripper opening/closing mechanism and a turn-over mechanism.
In an apparatus disclosed in Japanese Patent Publication No. 45-12929, gripper units corresponding to the gripper units (A) 25 and (B) 28 are fixed on pivot shafts, and pinions mounted on the pivot shafts are meshed with segment gears swung by a cam mechanism. A spring is hooked between the gripper unit and the cylinder to apply a pivotal force to the pivot shafts in a direction where a cam follower is urged against a cam surface. An additional cam mechanism is arranged to open/close the gripper unit at the turn-over position of the gripper unit.
With the above construction, when the gripper units come to the turn-over position, both the gripper units are pivoted through the segment gears and the pinions against the biasing force of the spring by means of the cam mechanism. When the turn-over operation is completed, the additional cam mechanism is operated to open/close the gripper units, thereby regripping the trailing edge of the sheet from one group of gripper units to the other group of gripper units. When the next regripping is timed, the gripper units are returned to the initial position by the biasing force of the spring.
In an apparatus disclosed in Japanese Patent Publication No. 51-7409, a turn-over mechanism includes segment gears, pinion gears, and springs as in the above mechanism. However, a gripper opening/closing mechanism of this apparatus is different from that described above. More specifically, the gripper opening/closing cam is swung at a predetermined timing by an additional cam, and the corresponding cam follower is reciprocally moved. The spring for closing the gripper unit is hooked between the free end portions of the gripper and the gripper pad.
In the conventional turn-over apparatuses having the constructions described above, a rotational force generated by a biasing force of the spring always acts on the pivot shafts. A large load acts on the teeth of the pinion fixed on the pivot shaft, and the teeth are easily worn out. In addition, since the gripper return spring is mounted on the pivot shaft, and a chain is connected to one end of the spring and is wound around the pivot shaft. The chain may be broken by the impact load of the cam. Furthermore, the gripper opening/closing cam and the gripper turn-over cam must be shifted when printing is switched between single side printing and double side printing. As a result, the construction and operations are complicated. In the apparatus disclosed in Japanese Patent Publication No. 51-7409, the gripper opening/closing cam is pivoted at a predetermined timing, and an excess load acts on the cam followers. As a result, the cam follower tends to be worn out. In addition, the biasing force of the gripper return spring always acts on the gripper pad shaft, and this shaft tends to be worn out.